In order to prepare foams through the use of blowing agents, nucleating agents are necessary to initiate bubbles which are large enough to continue growing, rather than collapse or redissolve. For large cell foams, e.g., those having cell diameters greater than 100 micrometers (.mu.m), small weight percent loadings of relatively large, insoluble, inorganic particulates, such as talc, frequently serve to initiate the required number of bubbles. However, to prepare a small cell foam (i.e., those foams having cell diameters less than 100 .mu.m), the number of nucleator particles must be increased in order to increase the number of cells in order for a given final density product to be achieved. The use of conventional nucleating agents is undesirable in forming small cell foams for two reasons: 1) the weight percent loading of nucleator becomes large in order to introduce the necessary number of particles; and 2) the particles are usually too large in their volume and thus fill a large proportion of the cells. These factors present in conventional nucleating agents can lead to problems in both (i) the processing of the materials, for example inhomogeneous blending of the nucleator or abrasive action on equipment, and (ii) the properties of the product foam, such as increased density or decreased insulation efficiency.
Smaller nucleators have been formed through the introduction of zinc stearate into the matrix polymer. The micelle-like aggregates formed by the salt function as nucleators, but the range of particle sizes of these nucleators can be quite broad, leading to nonuniform nucleation.
Small cell foams can also be formed by homogeneous nucleation (i.e., non-nucleated) at high pressures. However, this technique is dependent upon random fluctuations within the pressurized polymer matrix, which can be difficult to control and which are easily overwhelmed by fortuitous heterogeneous nucleators (i.e., unintended contaminates). Therefore, new reliable methods that overcome these shortcomings would be most useful.
In recent years, certain polymers, referred to as dense star polymers or dendrimers or as STARBURST.TM. (a trademark of Dendritech Inc.) polymers, have been developed. Dense star polymers or dendrimers exhibit molecular architecture characterized by regular dendritic branching with radial symmetry. These radially symmetrical molecules are referred to as possessing "starburst topology". These polymers are made in a manner which can provide concentric dendritic tiers around an initiator core. The starburst topology is achieved by the ordered assembly of repeating units, usually organic groups, in concentric, dendritic tiers around an initiator core; this is accomplished by introducing multiplicity and self-replication (within each tier) in a geometrically progressive fashion through a number of molecular generations. The resulting highly functionalized molecules have been termed "dendrimers" in deference to their branched (tree-like) structure as well as their oligomeric nature. Thus, the terms "dense star oligomer" and "dense star dendrimer" are encompassed within the term "dense star polymer". Also topological polymers, with size and shape controlled domains, are dense star dendrimers that are covalently bridged through their reactive terminal groups, which are referred to as "dense star bridged dendrimers." The term "dense star bridged dendrimer" is also encompassed within the term "dense star polymer".
These dense star polymers have been previously described as a solvent soluble, radially symmetrical dense star polymer wherein the dense star polymer has at least one core branch emanating from a core, said branch having at least one terminal group provided that (1) the ratio of terminal groups to the core branches is two or greater, (2) the density of terminal groups per unit volume in the polymer is at least 1.5 times that of an extended conventional star polymer having similar core and monomeric moieties and a comparable molecular weight and number of core branches, each of such branches of the extended conventional star polymer bearing only one terminal group, and (3) the dense star polymer has a molecular volume that is no more than about 60 to 80 percent of the molecular volume of said extended conventional star polymer as determined by dimensional studies using scaled Corey-Pauling molecular models, and has regular dendritic branching. (See, for example, the descriptions of dense star polymers in U.S. Pat. Nos. 4,507,466; 4,558,120; 4,568,737; 4,587,329; and 4,694,064; and European Patent Application Publication No. 0 271 180, the disclosures of which are hereby incorporated by reference.) It has been previously found that the size, shape and properties of these dense star polymers can be molecularly tailored to meet specialized end uses (e.g., European Patent Application Publication No. 0,271,180, the disclosure of which is hereby incorporated by reference.) However, nowhere is the use of such dense star dendrimers as nucleating agents for the production of small cell foams taught or suggested.